Latest Miocene to Quaternary Deformation in the Southern 10.1002/2015JB012135 Chaiwopu Basin, Northern Chinese Tian Shan Foreland

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Journal of Geophysical Research: Solid Earth

RESEARCH ARTICLE Latest Miocene to Quaternary deformation in the southern 10.1002/2015JB012135 Chaiwopu Basin, northern Chinese Tian Shan foreland

Key Points: Honghua Lu1, Zhen Wang1, Tianqi Zhang1,2, Junxiang Zhao3, Xiangmin Zheng1, and Youli Li4 • Stratigraphic and subsurface data indicate existence of growth strata 1School of Geographic Sciences, East China Normal University, Shanghai, China, 2Now at Institute of Earth and • New magnetostratigraphy constrains 3 the inception of growth strata at Environmental Science, University of Potsdam, Potsdam, Germany, Institute of Crustal Dynamics, China Earthquake 4 ~6.4 Ma Administration, Beijing, China, Key Laboratory of Earth Surface Processes of Ministry of Education, Peking University, • Quaternary deformation is revealed Beijing, China by geomorphologic data

Abstract Basinward propagation of fold and thrust belts is a crucial geological process accommodating Cenozoic crustal shortening within the India-Eurasia collision zone. Anticlinal growth strata in the southern Correspondence to: H. Lu, Chaiwopu Basin (a piggyback basin) of the northern Chinese Tian Shan foreland record basinward [email protected] encroachment of the Tian Shan along the Junggar Frontal Thrust Fault. A new magnetostratigraphic section constrains the onset of syntectonic growth strata at circa 6.4 Ma and suggests synchronous basinward Citation: thrusting and propagation of the Tian Shan. The intense alluviation in the southern Chaiwopu Basin ceased at Lu, H., Z. Wang, T. Zhang, J. Zhao, circa 0.55 Ma due to significant anticlinal growth and its resultant river incision. More recent anticlinal growth X. Zheng, and Y. Li (2015), Latest and deformation during the late Quaternary are revealed by folded river terraces developing across the Miocene to Quaternary deformation in H the southern Chaiwopu Basin, northern anticline. The terrace height profile indicates that terrace T1 has been vertically offset about 0.6 m by thrust Chinese Tian Shan foreland, J. Geophys. faulting since its formation at about 7 Ka. The stratigraphic and geomorphic data presented in this work are – Res. Solid Earth, 120, 8656 8671, helpful to understand the initiation of thrust-related folding, as well as aggradation and subsequent incision, doi:10.1002/2015JB012135. in foreland basins of the Tian Shan in relation to the India-Asia collision. Received 20 APR 2015 Accepted 13 NOV 2015 Accepted article online 17 NOV 2015 1. Introduction Published online 12 DEC 2015 Since the early Cenozoic, the India-Asia collision has driven deformation exerting a dominant control on the tectonic and topographic patterns of central Asia, including western China [Najman et al., 2001; Zhang, 2004] (Figure 1a). As a result of the collision, the Tian Shan (Shan means “mountains” in Chinese) has been tectonically reactivated and intensely uplifted as crustal shortening propagated into its foreland basins [Windley et al., 1990; Avouac et al., 1993; Yin et al., 1998; Deng et al., 2000]. The well-exposed rocks in and adjacent to the arid to semiarid Tian Shan provide an ideal natural laboratory where the temporal and spatial history of tectonic deformation caused by the India-Asia convergence can be unraveled with exceptional clarity. Previous studies of the timing and magnitude of the Cenozoic tectonic deformation of the Tian Shan have utilized low-temperature thermochronology [e.g., Hendrix et al., 1994; Bullen et al., 2003; Sobel et al., 2006a, 2006b; Lu et al., 2013a; Yu et al., 2014], magnetostratigraphy [e.g., Chen et al., 2002, 2007; Charreau et al., 2005, 2006; Heermance et al., 2007, 2008; Sun et al., 2009; Lu et al., 2010a; Thompson et al., 2015], geological and geodetic investigations [e.g., Reigber et al., 2001; Zubovich et al., 2010], or geomorphologic data [e.g., Avouac et al., 1993; Burchfiel et al., 1999; Fu et al., 2003; Scharer et al., 2006; Lu et al., 2010b; Yang et al., 2012; Li et al., 2013]. Nonetheless, the detailed character, sequence, and magnitude of the structural, depositional, and geomorphic responses to the ongoing collision contain many gaps. Here we provide new stratigraphic and geomorphic data on the initiation of thrust-related folding, as well as aggradation and subsequent incision within a piggyback basin (i.e., the Chaiwopu Basin) of the northern Chinese Tian Shan foreland. The Chaiwopu Basin is situated in the easternmost part of the northern Chinese Tian Shan foreland (Figures 1a and 1b). Geomorphologically and tectonically, the basin lies in the transitional zone of the eastern Tian Shan (Bogda Shan and Balikun Shan) and the western Tian Shan: a diffuse N-S boundary which generally lies near the city of Urumqi (Figures 1a and 1b). Given its position at this transition, the Chaiwopu Basin is thus a key area to understanding the late Cenozoic deformation of the Tian Shan. At the southern margin of the basin, the small-scale Saerqiaoke anticline dominates the local topography [Lu et al., 2014] (Figures 1b and 1c). This anticline is causally linked with basinward thrusting along the north branch of southern Chaiwopu

©2015. American Geophysical Union. Fault (NSCF), which branches from the Junggar Frontal Thrust Fault (JFTF) bounding the northern Tian All Rights Reserved. Shan and its foreland basin [Liu et al., 2007]. However, the history of anticlinal growth and deformation

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Figure 1. (a) Map shows tectonic setting and topographical pattern of the Indian-Eurasian convergence zone. (b) Digital elevation model of the northern Chinese Tian Shan foreland where fold and thrust belts I to III characterize the regional topography. (c) Geological map in the southern Chaiwopu Basin where thick Quaternary alluviums deposited as alluvial fans F1 and F2 of the Urumqi River. Circles with numbers show the sampling sites for optically stimulated luminescence (OSL) dating, as reported in Table 1.

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remains unclear, with little stratigraphic and geomorphic evidences to precisely date the late Cenozoic tectonic activity in the southern Chaiwopu Basin. In foreland basins, detailed sedimentological and geomorphological investigations are helpful to understand the tectonic evolution of the basin [e.g., Molnar et al., 1994; Burbank et al., 1996; Burchﬁel et al., 1999; Burbank and Anderson, 2011; Yang and Li, 2011; Yang et al., 2014; Thompson et al., 2015]. The main aim of this work is to develop a chronology of the late Cenozoic deformation in the southern Chaiwopu Basin of the northern Chinese Tian Shan foreland by integrating stratigraphic and geomorphic data. Our results help to reveal the activity of major faults in the basin and further to understand their seismic behavior: a focus that is signiﬁcant given the active deformation and high seismicity in the modern Tian Shan and its foreland basins [Deng et al., 2000; Reigber et al., 2001].

2. Geological Background As one of the largest and most active orogenic belts in the Asia inland, the EW-trending Tian Shan has experi- enced a complex geological history. Previous studies [e.g., Windley et al., 1990; Yin et al., 1998; Deng et al., 2000] have revealed that the ancestral Tian Shan arose from several block collisions during the Late Devonian-Early Carboniferous and Late Carboniferous-Early Permian, along with the formation of a suite of EW-trending strike-slip and thrust faults (Figure 1a). The Mesozoic deformation of the range was dominated by relative tectonic stability during the Triassic-Late Jurassic, followed by active deformation during the Late Jurassic-Early Cretaceous [Lu et al., 2010a]. The subsequent relative stability during the latest Mesozoic- Paleogene caused the beveling of topography within the Tian Shan [Allen et al., 1991; Bullen et al., 2003]. As a result of the Cenozoic India-Asia collision, the Tian Shan range has been tectonically reactivated and gradually encroached into its foreland basins [Deng et al., 2000; Lu et al., 2010a, 2013a]. In the northern Chinese Tian Shan foreland, such a geological process has formed three fold and thrust belts known as belts I to III that characterize the regional topography (Figure 1b). Situated in the easternmost part of the northern Chinese Tian Shan foreland, the Chaiwopu Basin is separated from the Junggar Basin to the northwest by the Xi Shan (i.e., the easternmost part of fold and thrust belt I) (Figure 1b), a low mountain range with elevations of 1–2 km. The basement rocks of the Chaiwopu Basin com- prise Permian to Triassic strata [Gao, 2004]. The basin ﬁll consists of Tertiary and Quaternary strata. The Tertiary comprises the Donggou Group (Cretaceous-Eocene) and the Manas (Oligocene), Qianshan (Miocene), and Changjihe (Pliocene) Formations, and the lithology is dominated by lacustrine mudstone, sandstone, and con- glomerate. In general, the basement rocks and Tertiary strata are exposed on both the southern and northern margins of the Chaiwopu Basin due to tilting, thrusting, and folding. The overlying Quaternary strata are massive alluvial conglomerates. Where the Urumqi River ﬂows into the lowland southern Chaiwopu Basin,

two episodes of Quaternary alluvial fan deposition (the fans F1 and F2)areapparent[Lu et al., 2014] (Figure 1c). Stratigraphically, the fan F1 conglomerates (designated as Saerqiaoke Gravel by Zhou et al. [2002]) is equivalent to Urumqi Gravel, an early-middle Pleistocene succession of thick ﬂuviatile conglomerates deposited

as a part of the basin ﬁll in the northern Chaiwopu Basin [Gao, 2004]. Progressive incision into the fans F1 and F2 by the Urumqi River has created a well developed ﬂight of terraces [Lu et al., 2014].

3. Methodology In a fault-bounded foreland basin like the Chaiwopu Basin, the investigations of both tectono-sedimentary evolution and geomorphic expression of tectonic deformation are crucial to reveal the history of tectonic deformation of the basin [e.g., Burbank, 1992; Burbank et al., 1996; Bullen et al., 2003; Chen et al., 2007; Heermance et al., 2007; Lu et al., 2010a; Burbank and Anderson,2011;Vandenberghe et al., 2011; Yang et al., 2012, 2014]. Certainly, such analysis should be placed in a robust temporal framework. We thus conducted a magnetostratigraphic investigation in order to date the basin ﬁll. Correspondingly, our work comprises two principal facets: (i) stratigraphic and magnetostratigraphic analysis and (ii) geomorphologic investigation.

3.1. Stratigraphic and Magnetostratigraphic Analysis The continuous growth and uplift of a structure (such as a fault-related fold) are capable of causing a distinct geometry of sediments progressively depositing above this structure, and correspondingly, the stratigraphy is described as growth strata (i.e., syntectonic sedimentation) [e.g., Suppe et al., 1992; Burbank et al., 1996;

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Figure 2. Measured geological section across the Saerqiaoke anticline exposed by the Urumqi River. (a–c) The representative lithofacies. “~278–283 m” shows the stratigraphic level in the measured Urumqi River section (URS). The ﬁrst signiﬁcant change in dip angle in the URS is interpreted to represent inception of syntectonic growth strata. The labeled fault in the URS is a branch of the north branch of southern Chaiwopu Fault (NSCF).

Heermance et al., 2007]. Thus, we reconstructed the local tectonic evolution by studying the geometry of the sedimentary strata in the southern Chaiwopu Basin, and this reconstruction is based mainly on the following two lines of investigations. In the field, we extensively investigated the stratigraphic contacts, took detailed measurements of bedding attitudes, and described the lithofacies. We also analyzed a seismic reflection profile through the Saerqiaoke anticline which was reported by Liu et al. [2007]. In order to date the basin fill, we sampled a magnetostratigraphic section in the upper Cenozoic terrigeneous sequence, exposed across the southern limb of the Saerqiaoke anticline by the Urumqi River (Figure 1c) and hereafter referred to as the “Urumqi River section (URS)”. The sampled URS is about 310 m thick (Figure 2). Lithologically, the section comprises mainly brownish yellow/red mudstone, but the middle part of the section is dominated by interbedded brown/gray conglomerates and mudstone (Figure 2). This lithofacies association is interpreted to define an alluvial-lacustrine depositional system, a sedimentary environment developing in the northern Chinese Tian Shan foreland during the periods of Neogene-Quaternary [e.g., Charreau et al., 2005; Lu et al., 2010a]. The absence of significant observable faulting or major erosional discordance suggests no significant depositional breaks in the URS, i.e., a relatively continuous sedimentary process.

3.2. Geomorphologic Investigation Deformed river terraces and alluvial fan surfaces can be used to reconstruct the recent tectonic evolution in the basin margin [e.g., Molnar et al., 1994; Burchfiel et al., 1999; Burbank and Anderson, 2011; Yang and Li, 2011]. The fluvial geomorphologic classification in the southern Chaiwopu Basin has been reported in our recent work [Lu et al., 2014]. In this paper, we determine the timing and magnitude of the anticlinal deformation by characterizing the deformed terraces across the Saerqiaoke anticline. The longitudinal profiles of terraces are used to reveal the Quaternary deformation in relation to the fold growth. Such profiles were H extracted from Google Earth images except the terrace height profile of T1 that was measured with TruPulse 200. The ages of the high terraces are from our previous study [Lu et al., 2014], and the formation ages of the low terraces reported in this paper were dated by optically stimulated luminescence (OSL) dating method. Samples for OSL dating were taken by inserting a 20 cm long, 5 cm diameter steel pipe with one end covered with opaque materials into the sampled layer. In order to ensure maximal shielding, only the middle part of each sample was analyzed. The OSL dates were determined in the Institute of Crustal Dynamics of

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China Earthquake Administration following the procedures of Rees-Jones [1995] and Wang [2006]. We have also investigated the geomorphology of the area beyond the Saerqiaoke anticline to identify the transition of downcutting and deposition of the Urumqi River.

4. Magnetostratigraphy of the Urumqi River Section 4.1. Paleomagnetic Sampling and Lad Measurements In the Urumqi River section, paleomagnetic sampling yielded 82 sampling horizons from which a total of 184 specimens were collected. We drilled specimens with an average interval of about 3.7 m varying between 0.2 and 6.5 m. The specimens were collected from mudstone, siltstone, or fine-grained sandstone. The orienta- tions of all the specimens were measured with a magnetic compass corrected by the local magnetic declination (4.8°E at the URS sampling site). One specimen per sampling horizon was subjected to stepwise (13–17 steps) thermal demagnetization. For the stratigraphic horizons where measured specimens showed unstable high-temperature demagnetization trajectories, a second specimen was selected for another round of thermal demagnetization. Generally, thermal demagnetization intervals are 50–80°С below 500°С, 25°С or 35°С between 500 and 610°С, and 10–20°С above 610°С. Magnetic remanence was measured with a 2G-755 cryogenic magnetometer installed in magnetic-free space (<300 nT) at the Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing. The characteristic remanent magnetization (ChRM) directions of the measured specimens were determined by principal component analysis [Kirschvink, 1980], and the identified directions were further analyzed with Fisher statistics [Fisher, 1953]. The directions (magnetic declination and inclination) of the selected specimens were used to calculate virtual geomagnetic pole (VGP) latitudes. The magnetic polarity stratigraphy (MPS) was finally constructed using the VGP data for the sampled URS and then correlated to the geomagnetic polarity time scale (GPTS) of Lourens et al. [2004] to constrain the age of the URS.

4.2. Demagnetization The natural remanent magnetization is between 0.381 × 10-5 A/m and 12.0 × 10-5 A/m with an average of 3.26 ± 2 × 10-5 A/m. Stepwise thermal demagnetization successfully isolated dominant magnetic components for most of the analyzed specimens after eliminating the viscous remanent magnetization, whereas a few specimens exhibited scattered demagnetization directions which we did not use for the subsequent analysis. For the specimens exhibiting relatively stable high-temperature demagnetization trajectories, the maximum angular deviation was between 4.5° and 14.9° with an average of 12.4 ± 4.4°. The decay curves during demagnetization and Zijderveld plots indicate that their demagnetization behaviors contain three main magnetic components (Figure 3): the low-temperature component (LTC) between 0°С and 250–300°С representing ~30–40% of the total remanence, the intermediate temperature component (ITC) between 250–300°С and 500–525°C representing ~25–40% of the total remanence, and the high- temperature component (HTC) in a temperature range extending from ~500 to 525°С to commonly 610 or rarely 680°С representing ~20–45% of the total remanence. For the HTC of the most specimens, the magnetic carrier is most likely magnetite or titanomagnetite, as indicated by the sharp decrease in remanence occurring between 525 and 580°C (Figures 3a, 3b, and 3c). The HTC of a few specimens is likely carried by hematite or maghemite that revealed by the signiﬁcant drop in remanence between 610 and 680 C (Figure 3d) [Butler, 1992].

4.3. Reversal and Fold Tests Reversal and fold tests are commonly used to determine whether acquisition of the ChRM directions is predeformation, syndeformation, or postdeformation. Figure 4 shows that tilt correction improves the group- ing of ChRM poles of the URS. The Fisherian site-averaged normal and reversed polarity poles have an observed angular difference of 5.98°. This angular difference is smaller than the critical angle of 12.11° at the 95% conﬁdence level and on the basis of the assumed identical dispersion К, implying that the URS passes the C-class reversal test [McFadden and McElhinny, 1990]. Due to poor outcrop preservation, only a few oriented specimens were collected from the northern limb of the Saerqiaoke anticline to perform the standard fold test. Instead, the simple fold test of McElhinny [1964] was employed with the specimens from

the URS in the fold’s southern limb (Figure 4). The results show that the Кcrit values of 1.60 (normal polarity) and 1.47 (reversed polarity) are respectively smaller than the Кs/Кg ratios of 1.64 (normal) and 1.53 (reversed)

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Figure 3. Representative thermal demagnetization plots typically showing two or three magnetic components. The low-temperature component (LTC), the intermediate temperature components (ITC), and the high-temperature component (HTC) of the characteristic remanence magnetization (ChRM) are depicted for each selected sample. (left column) Remanent magnetic intensity graphs; (center column) Zijderveld diagrams; and (right column) Equal-area projections. 300 = 300°C. “63.3 m” indicates the sampling stratigraphic level of the measured specimen in the Urumqi River section.

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Figure 4. Stereonet plots of palaeomagnetic data from the URS. Small solid (inclined down) and white (inclined up) circles indicate individual measurements with the larger circles showing the α95 error around the Fisher mean. The Fisher mean data are listed adjacent to each plot.

at the 95% confidence level. Here Кs and Кg are the dispersions in the stratigraphic and geographic coordinates, and Кcrit is defined as the f distribution with 2(NÀ1) degrees of freedom, where N is the amount of specimens with a specified magnetic polarity. The above results suggest a positive fold test [McElhinny, 1964]. Thus, the positive reversal and fold tests (Figure 4) imply that the ChRM directions of the URS sediments were acquired before deformation.

4.4. Magnetic Polarity Stratigraphy The ChRM directions of all the accepted specimens were determined by least squares fitting through the temperature points commonly above 450°С (minimum of three but more typically four to six). The determined ChRM directions were then used to calculate the VGP latitudes. When constructing the MPS for the URS, specimens with VGPs <30° were not used except when they displayed the same polarity with the adjacent specimens showing VGPs ≥30°. However, a single specimen with VGP ≥30° was not considered if its adjacent specimens display the opposite polarity. The ChRM vector directions of a total of 64 specimens were used to define the MPS of the URS, which contains 13 magnetozones (Figure 5). Each magnetozone is defined by at least two specimens with VGPs ≥30°. Two zones (n1 and n2) are less certain because they include only one specimen with VGP ≥30°. Overall, five normal (N1-N5) and four reversed (R2-R5) magnetozones are clearly identified in the URS column. Our magnetostratigraphy was then correlated to the GPTS of Lourens et al. [2004] (Figure 5) based on the following observations: (1) one interval with a distinctive pattern of local magnetic polarity (N1-N4), which is interpreted as the Gilbert polarity chron; (2) one relatively long reversed magnetozone (R5); (3) the relatively continuous depositional process throughout the section without significant depositional breaks; and (4) the possible age of Mio-Pliocene of the sampled URS sediments. On the basis of the vertebrate fauna containing Equus sanmeniensis (horse) as well as the preliminary magnetostratigraphic result, the upper boundary of Urumqi Gravel in the northern Chaiwopu Basin is inferred to lie slightly above the Brunhes-Matuyama paleomagnetic boundary (i.e., <0.78 Ma) [Gao, 2004]. Its lower boundary is inferred to be not older than about 1.87 Ma [Li et al., 1990; Gao, 2004]. Stratigraphically, Urumqi Gravel corresponds to Saerqiaoke Gravel

(i.e., the fan F1 conglomerates) in the southern Chaiwopu Basin, implying that these two stratigraphic units have a similar age. Detailed ﬁeld observations show that the relatively gently inclined layers of Saerqiaoke Gravel overlie the sampled URS sediments (see Figure 10b). This observed superposition suggests the URS is likely upper Neogene. Overall, our preferred correlation matches the main patterns of magnetic polarity reversal in the Latest Miocene to Pliocene GPTS [Lourens et al., 2004] (Figure 5). Of course, some ambiguities remain with this correlation, e.g., normal magnetozones n1 and n2. We matched n1 and n2 with the subchrons C2An.3n

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Figure 5. Magnetic polarity stratigraphy (MPS) of the Urumqi River section. (a) Lithology log with the base of growth strata dated at about 6.4 Ma. (b) Magnetic declination in tilt-corrected coordinates (shaded zone equals 180°–360°). (c) Magnetic inclination in tilt-corrected coordinates (shaded zone equals ±45°). (d) Virtual geomagnetic pole (VGP) latitude (shaded zone equals ±30°). Open circles represent specimens with absolute values of VGPs <30°, which were not used to define the MPS of the URS, except for the local magnetozones n1 and n2. (e) Magnetic polarity stratigraphy defined commonly by specimens with absolute values of VGPs ≥30°. Black and white zones indicate normal and reversed polarity, respectively. Boundaries between magnetozones are defined by the midpoint between two specimens with opposite polarity. Magnetozones (n1 and n2) defined only by two specimens with one showing VGP <30° are displayed by short-half bars. (f) Reference geomagnetic polarity time scale (GPTS) of Lourens et al. [2004].

and C3An.2n, respectively, based mainly on the correlation of the magnetozones N1-N5 with the subchrons C3n.1n-C3An.1n (Figure 5). Despite inevitable uncertainties, the correlation, especially between the magnetozones N1-N5 and the subchrons C3n.1n-C3An.1n of the GPTS [Lourens et al., 2004], appears acceptable. Our above correlation thus constrains the basal age of the sampled URS to about 6.8 Ma, spanning from ~6.8 to ~3.3 Ma (Figure 5). Based on this correlation, the relationship between the thickness and magnetostratigraphic ages of the URS sediments shows two decreases in the sediment-accumulation rate at about 6.1 and 4.1 Ma (Figure 6), with an average rate of ~95 m/Myr over the 3.5 Myr interval of the URS.

5. Geomorphologic Observations

In our recent work, nine terraces (T1 to T9, increasing systematically in elevation) have been identiﬁed in the southern Chaiwopu Basin (Figure 7a) [Lu et al., 2014]. Field investigations suggest that terraces T9, T7, T5,and T4 are the best expressed ﬂuvial features in this area, and terraces T9 and T4 correspond to alluvial fans F1 and F2, respectively. Above the core of the Saerqiaoke anticline, terrace T1 can be divided to the high terrace H L T1 and the low terrace T1 based on the terrace height above the riverbed (Figures 1c and 7a). Our new OSL

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Figure 6. Age versus thickness plot of the Urumqi River section showing changes in sediment-accumulation rate at about 6.1 and 4.1 Ma, respectively.

H L dating data (Table 1) constrain the formation age of terraces T1 and T1 to ~7 Ka and ~2 Ka, respectively. The formation age or depositional age of the older terraces have been determined by electron spin resonance

dating method [Lu et al., 2014]: the abandonment of terrace T9 (i.e., fan F1) was dated at about 0.55 Ma; the depositional age of terrace T7 was constrained to about 0.26 Ma; and terrace T5 is estimated to have formed at about 0.14 Ma.

About 10 km northeast of the distal end of F1 (Figure 1c), the downcutting into the alluviums of fan F2 has yielded four terraces well developed along the course of the Urumqi River (Figures 7c to 7f). Based mainly on surface characteristics, elevations above the modern riverbed, terrace deposits and the depositional

age of the lowest terrace (about 6 Ka, Table 1), these four terraces were classified as T1 to T4 (Figure 8a). It is clear that the terrace deposits of the three younger terraces (T1 to T3) overlie the fan F2 conglomerates (Figures 7c to 7f and 8a). H Here we focus on the deformation of terraces T7, T5, T4, and T1 (Figures 7b and 8b). In response to significant rock uplift of the fold, terraces T7, T5, and T4 have been created as strath terraces [see Lu et al., 2014, Figure 5]. The longitudinal profiles indicate that these three terraces have been significantly warped up over the Saerqiaoke anticline (Figure 8b). In more extensive areas beyond this anticline, old terraces (here they are

terraces T7 and T5) have been buried due to accumulation of younger alluvial sediments caused by relative subsidence (Figure 8b). Subsequent river incision has formed young ﬁll terraces (like T4 to T1) in these areas. Here it is unlikely to quantitatively estimate the magnitude of deformation of terraces T7 and T5 due to the burial (Figure 8b). It is clear that, however, the older terraces record more deformation (Figure 8b). The more H recent deformation of the Saerqiaoke anticline is recorded by the young terrace T1 (Figures 1c and 7b). In the anticlinal core, a branch of NSCF controlling the fold growth, which has the dip direction of ~215°

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H Figure 7. (a) Alluvial sequence developed by the Urumqi River in the southern margin of the Chaiwopu Basin. (b) The deformed terrace T1 , with the insert showing the magnitude of thrust faulting of about 60 cm determined by the measured terrace height proﬁle. (c–f) Photos showing the geomorphic structure of four terraces that developed owing to the downcutting into the alluviums of fan F2 about 10 km northeast of the distal end of F1. In Figures 7a and 7f, the inserts show the sections from which the OSL dating samples were taken. See Figure 1c for the locations and view directions of these photos.

H and the dip angle of ~62°, reaches the surface between the terrace deposits of T1 and the underlying strata of Neogene and upper Pleistocene on the east bank of the Urumqi River (Figure 7b). It has thrust the Neogene H mudstone northward onto the upper Pleistocene conglomerates and has offset the terrace surface of T1 . H The measured terrace height proﬁle transverse to the thrust scarp indicates that terrace T1 has been vertically offset about 0.6 m by the thrust (Figure 7b).

6. Discussion 6.1. Northward Propagation of the Tian Shan at ~6.4 Ma In foreland basins, variations in lithofacies and rates of sediment accumulation could reveal local tectonic activity [e.g., Meigs et al., 1995; Lu et al., 2010a]. However, these changes could also be forced by nontectonic factors, including source lithology, transport distance [DeCelles and Giles, 1996], and paleoclimate [Molnar, 2001, 2004; Zhang et al., 2001; Dupont-Nivet et al., 2007]. Thus, the sediments, such as conglomeratic facies, cannot serve as deﬁnitive records of tectonic deformation within a foreland basin. In contrast, growth strata, i.e., syntectonic sedimentation, can indicate a direct link to growth and uplift of a structure, such as a fault-related fold [Riba, 1976; Suppe et al., 1992; Burbank et al., 1996]: its continuous growth can cause a characteristic wedge-shaped, syntectonic geometry of the sediments depositing over the structure [Burbank et al., 1996]. Field observations on stratigraphic geometry of Neogene-Pleistocene (Figures 2, 9, and 10b) reveal the existence of growth strata that reﬂect the syndepositional growth of the Saerqiaoke anticline in relation to the NSCF. Systematic changes in bedding dips were observed on both limbs of the Saerqiaoke anticline. In the fold’s southern limb, the Neogene strata close to the anticlinal core dip to the southwest at an angle >30°

(Figures 9a and 10b). The attitude of the overlying F1 conglomerates (i.e., Saerqiaoke Gravel) is concordant with the Neogene strata (Figure 10b). About 4 km to the south, the dips of Saerqiaoke Gravel exposed in the Urumqi River valley decrease to <10° (Figures 9b and 10b). Wherever these two stratal units are juxta- posed close to the anticlinal core in Saerqiaoke’s northern limb, the dip angle of Saerqiaoke Gravel gradually decreases upward from near vertical in the Urumqi River valley (Figures 9c and 10b) to ~45° near the top of

LU ET AL. NORTHERN TIAN SHAN FORELAND DEFORMATION 8665 Journal of Geophysical Research: Solid Earth 10.1002/2015JB012135 b the alluvial fan F1 (Figures 9d and 10). Steeper strata were observed on the forelimb of this fold (Figures 2 and 10b), indicating its asymmetric structure. The measured URS in Saerqiaoke’s southern limb further reveals the geometry of sedimentary strata composing the Saerqiaoke anticline. In general, the stratal dips in the URS gradually decrease upward from about 80° at the base of the section (close to the anticlinal core) to about 45° at its top (away from

(Gy/kyr) OSL Age (ka) the anticlinal core) (Figures 2 and 10b). Annual Doses The above observations on stratal attitudes indicate a characteristic wedge-shaped stratigraphic geometry of the strata composing Saerqiaoke’s limbs (Figure 2 and 10b), consistent with syntectonic growth strata in relation to Equivalent Doses (Gy) fold growth [Burbank et al., 1996]. This interpretation on growth strata is based mainly on the observed stratigraphic contact relationship and changes in stratigraphic attitude. A true interpretation should be further evidenced

Water by high-resolution subsurface data for this fold. Our new m). μ Content (%) magnetostratigraphic study constrains the deposition of 11 – growth strata on the backlimb of the Saerqiaoke anticline at circa 6.4 Ma (Figure 5), suggesting the initiation of the anticlinal growth around the same age. More detailed insight on the anticlinal growth emerges from analysis of the sediment-accumulation rate curve of the URS g/g) K (%) μ (Figure 6). As shown in Figure 6, the magnitude of the reduction in accumulation rate at ~6.1 and ~4.1 Ma is striking: about 15% and 50% decreases, respectively. In general, reduction of sediment accumulation across the g/g) Th ( μ 1.36 6.71 2.35 0.34 3.77 21.98 ± 1.46 5.83 ± 0.70 2.52 10.6 2.33 3.36 4.64 35.76 ± 1.66 7.70 ± 0.85 2.43 12 2.20 2.57anticlinal 4.65 crest 10.57 ±would 0.50 be 2.28 ± 0.25 expected in response to acceler- ating crestal uplift [Burbank et al., 1996]. Given the position of the URS close to the anticlinal core (Figure 10b), we . a 1

T tentatively interpret the reductions of accumulation rate at ~6.1 and ~4.1 Ma observed in the URS as indicative of synchronous signiﬁcant folding and uplift of the Saerqiaoke anticline. ne sand and ﬁ . H . The interpreted ~6 Ma tectonic event in the southern 1 L 1 T

T Chaiwopu Basin is nearly synchronous with deformation observed in the other sites of the Tian Shan [e.g., Huang et al., 2006; Wang et al., 2008; Sun et al., 2009; Lu et al.,

ne sand of 2010a], the Pamir [e.g., Thiede et al., 2013; Thompson ﬁ et al., 2015], the Kunlun Shan [e.g., Zheng et al., 2000; Sun et al., 2008], and the northeastern Tibetan Plateau [e.g., silt very

thick terrace gravels of Fang et al., 2004, 2007; Li et al., 2014]. In foreland basins of the Tian Shan, acceleration of sediment-accumulation . σ height above the riverbed is about 8.4 m. rate and growth strata [Huang et al., 2006; Sun et al.,

At the base of ~90 cm thick silt above the ~1.9 m 2009; Lu et al., 2010a] and structural analysis [Wang et al., the terrace height above the riverbed is about 12 m. The sampled layer is ~20 cm thick very The terrace gravels is about 2.2 m thick and the terrace 2008] likewise identify the ~8–6 Ma uplift of the Tian

The terrace height above the current riverbed is about 19.6 m. Shan range and its resultant basinward encroachment. In the northern Kunlun Shan foreland, lithofacies changes

Calculated Values of Equivalent Doses, Annual Doses, and OSL Ages and an increase in sediment-accumulation rate occurring at circa 4.5 Ma are regarded as indicative of synchronous The age uncertainty is 1 The dating technique is Sensitivity-corrected Multiple Aliquot Regenerative-dose, and the dating material is quartz (4 Locations of dating samples and sampled sections are shown in Figures 1, 7a, and 7f, respectively.

a b c uplift of the northwestern Tibetan Plateau [Zheng et al., c 2 At the depth of 40 cm in the 80 cm thick 3 At the depth of 4.3 m in the ~4.6 m thick terrace gravels of Sample No.1 Sampled Layer (m) U ( Table 1. 2000]. A similar result from Sun et al. [2008] constrains

LU ET AL. NORTHERN TIAN SHAN FORELAND DEFORMATION 8666 Journal of Geophysical Research: Solid Earth 10.1002/2015JB012135

Figure 8. (a) Terrace to river cross section III-III’ of the Urumqi River based on the field investigations. Note that terrace deposits of all the three younger terraces T1 to T3 overlie the conglomerates of fan F2. See Figure 1c for the location of this section. (b) The longitudinal profiles of terraces T7 (IV-IV’), T5 (V-V’), and T4 (VI-VI’) (modified from Lu et al. [2014]) displaying sequential increase of deformation. See Figure 1c for the locations of these terrace longitudinal profiles.

the tectonic uplift of the Kunlun Shan at circa 5.3 Ma by magnetostratigraphic data on growth strata. On the northern front of the Qilian Shan in the northeastern Tibetan Plateau, the development of syntectonic deposition (growth strata) implies the growth and uplift of the range at circa 8 and 6.6 Ma [Fang et al., 2004; Li et al., 2014]. These above studies suggest that the ~8–6 Ma tectonic uplift is likely a regionally extensive, rather than a local event in the Asian interior. More deformation ages throughout the mountain belt help to better constrain this regional event. When considering that deformation in the Asian interior has progressively propagated into foreland areas through time, the ~8–6 Ma tectonic uplift should be a part of the overall propagation pattern. The uplifted topography resulting from the ~8–6 Ma event might have blocked the moisture transport of Westerlies and Indian Monsoon [e.g., Miao et al., 2012] and thus have contributed to stepwise aridiﬁcation in the Asian interior since ca. 6 Ma [e.g., Miao et al., 2012; Lu et al., 2013b].

6.2. Quaternary Deformation Revealed by Geomorphic Data Subsequent growth and uplift of the Saerqiaoke anticline can be inferred from the intense accumulation in the southern Chaiwopu Basin. Basinward thrusting and propagation of the Tian Shan along the JFTF could have caused the uplifted topography of the Urumqi River catchment basin relative to the lowland Chaiwopu Basin [Lu et al., 2014]. As a result, high-energy river systems enhanced erosion and transport. Vast amounts of clastic sediments were transported by the Urumqi River from the range into the southern

LU ET AL. NORTHERN TIAN SHAN FORELAND DEFORMATION 8667 Journal of Geophysical Research: Solid Earth 10.1002/2015JB012135

Figure 9. Photos showing the systematic changes in the stratigraphic attitudes across the Saerqiaoke anticline in the southern Chaiwopu Basin. Note that the alluviums in Figure 9e, dip approximately to the north with angle of about 5° but in Figure 9f where the elevation decreases by about 100m in comparison with the site of Figure 9e, the dip angle increases to about 12°. See Figure 1c for the locations and view directions of photos

Chaiwopu Basin and formed the mega alluvial fan F1 (Figure 1c). The accumulation of alluvial sediments of F1 ended at about 0.55 Ma due to river incision [Lu et al., 2014]. Such incision has been regarded as a response to growth and rock uplift of the Saerqiaoke anticline caused by northward thrusting of the NSCF (Figure 10) [Lu et al., 2014]. Therefore, we suggest anticlinal growth in the southern Chaiwopu Basin at circa 0.55 Ma. It is

worth noting the change in attitudes of the alluviums in the proximal part of fan F1, where the alluvium dips to the north in contrast to the stratigraphic attitudes in the southern limb of the Saerqiaoke anticline, with the lower (older) alluviums displaying steeper dips (Figures 9e, 9f, and 10b). Given that original attitude of alluvium deposited along mountain front is commonly characterized by a relatively gentle dip of a few degrees, this observation on bedding attitudes (Figures 9e, 9f, and 10b) seems to indicate the activation of the JFTF when the

alluviums of F1 deposited. The more recent deformation of the Saerqiaoke anticline is revealed by the deformed terraces. As described in section 5, the younger terraces record less deformation (Figure 8b). Undoubtedly, this geometry implies

continuous anticlinal growth since the abandonments of terraces T7, T5, and T4. More detailed insight on H deformation is from analysis of the young terrace T1 above the core of the Saerqiaoke anticline H (Figures 1c and 7b). The terrace height profile indicates that terrace T1 has been vertically offset about 0.6 m by the thrust since its formation at about 7 Ka, defining the rate of deformation of 0.09 mm/yr. In comparison with the Holocene vertical rate of ~1 ± 0.3 mm/yr on the Tugulu-Dushanzi thrust in the western part of the northern Chinese Tian Shan foreland [Avouac et al., 1993], this rate estimation is likely to indicate the relatively low activity of the NSCF during the Holocene in the southern Chaiwopu Basin, a piggyback basin in the easternmost part of the foreland. More geomorphologic data are needed to further constrain the tectonic activity in this area. Basinward propagation of the Tian Shan along the JFTF is capable of resulting in a more steepened river gradi- ent leading to overall downcutting into the old alluviums and thus yielding younger terraces [Lu et al., 2010b]. Our field observations of the geomorphology (Figures 7c to 7f and 8a) support this idea. About 10 km northeast

of the distal end of F1 (Figure 1c), the terrace deposits of the three younger terraces (T1 to T3) overlie the conglomerates of fan F2 (Figures 7c to 7f and 8a), suggesting the transition from deposition to downcutting of the Urumqi River since the abandonment of the surface of fan F2. Therefore, we tentatively propose that formations of the three young terraces T3 to T1 were forced to some extent by the regional uplift in the southern Chaiwopu Basin due to basinward propagation of the Tian Shan. Certainly, climate change during glacial-interglacial cycles is also an important factor forcing river downcutting and terrace formation

LU ET AL. NORTHERN TIAN SHAN FORELAND DEFORMATION 8668 Journal of Geophysical Research: Solid Earth 10.1002/2015JB012135

Figure 10. (a) Deep seismic reflection profile I-I′ stretching from the Nan Shan northward the Xi Shan (left) and the geological interpretation (right) (modified from Liu et al. [2007]). The dipping southward faults and the detachment horizon RA, which is located at a depth of ~17–18 km (TWT: two-way traveltime 6.5 S) at the southern end of the seismic profile [Liu et al., 2007], compose a typical thrust nappe. RC is a deeper detachment horizon gently dipping southward. See Figure 1b for the location of this seismic reflection profile. (b) II-II′ geological cross section along the course of the Urumqi River in the southern Chaiwopu Basin. The upper-lower Pleistocene conglomerates were deposited as a wedge-shaped depositional unit with the maximum thickness of about 400 m at the fanhead of alluvial fan F1 [Zhou et al., 2002]. The thickness of the Neogene strata is unknown. The topography of this cross section is extracted from Google Earth images. The measured stratigraphic attitudes are shown at the measuring sites to exhibit the systematic changes in stratigraphic attitude in the southern Chaiwopu Basin. See Figure 1c for the locationofthissection.

[e.g., Molnar et al., 1994; Pan et al., 2003; Lu et al., 2010b, 2014; Owen et al., 2014]. Here we cannot eliminate the

possible contribution of climate change to formations of terrace T3 to T1. We aim to collect more data on the alluvial sequence in order to better understand the relationship between terrace formation, tectonic uplift, and climate variation in the present study area.

7. Conclusions In the southern Chaiwopu Basin of the northern Chinese Tian Shan foreland, the Urumqi River has incised deeply into the basement rocks and its overlying basin ﬁll, thus yielding well-exposed outcrops of late Neogene-Quaternary terrigenous strata and well-developed Quaternary alluvial sequences. The systematic change in bedding dips displays a characteristic wedge-shaped stratigraphic geometry of the strata compris- ing Saerqiaoke’s southern limbs, a small-scale structure developing at the distal end of the oldest alluvial fan

F1 of the Urumqi River. This observation is interpreted to indicate the existence of growth strata on the fold’s southern limb. A new magnetostratigraphic study dates the basal age of growth strata to about 6.4 Ma, when folding of the Saerqiaoke anticline commenced due to basinward propagation of the Tian Shan. The resulting intense alluviation in the southern Chaiwopu Basin ceased around 0.55 Ma in response to signiﬁcant uplift of the Saerqiaoke anticline and the resultant downcutting. Folded terraces record more recent tectonic defor- H mation of this fold. The terrace height proﬁle indicates that terrace T1 has been vertically offset about 0.6 m by thrust faulting since its formation at about 7 Ka. Basinward encroachment of the fold and thrust belts in both the northern and southern Tian Shan foreland is a crucial geological process accommodating Cenozoic crustal shortening within the India-Eurasia convergent system. The stratigraphic and geomorphic

LU ET AL. NORTHERN TIAN SHAN FORELAND DEFORMATION 8669 Journal of Geophysical Research: Solid Earth 10.1002/2015JB012135

data presented in this work are helpful to understand initiation of thrust-related folding in the northern Chinese Tian Shan foreland in relation to such a geological process. Future work will focus on quantiﬁcation of fold growth rates using geologic and geomorphic data in the present study area.

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